Dual phase galvanized steel strip is made utilizing a thermal profile involving a two-tiered isothermal soaking and holding sequence. The strip is at a temperature close to that of the molten metal when it enters the coating bath.
Prior to the present invention, the galvanizing procedure whereby steel strip is both heat treated and metal coated has become well known and highly developed. Generally a cold rolled steel sheet is heated into the intercritical regime (between Ac1 and Ac3) to form some austenite and then cooled in a manner that some of the austenite is transformed into martensite, resulting in a microstructure of ferrite and martensite. Alloying elements such as Mn, Si, Cr and Mo are in the steel to aid in martensite formation. Various particular procedures have been followed to accomplish this, one of which is described in Omiya et al U.S. Pat. No. 6,312,536. In the Omiya et al patent, a cold rolled steel sheet is used as the base for hot dip galvanizing, the steel sheet having a particular composition which is said to be beneficial for the formation, under the conditions of the process, of a microstructure composed mainly of ferrite and martensite. The Omiya et al patent describes a galvanized dual phase product.
According to the Omiya et al patent, a dual phase galvanized steel sheet is made by soaking the cold rolled steel sheet at a temperature of 780xc2x0 C. (1436xc2x0 F.) or above, typically for 10 to 40 seconds, and then cooling it at a rate of at least 5xc2x0 C. per second, more commonly 20-40xc2x0 C. per second, before entering the galvanizing bath, which is at a temperature of 460xc2x0 C. (860xc2x0 F.). The steel, according to the Omiya et al patent, should have a composition as follows, in weight percent:
manganese, chromium and molybdenum should have the relationship:
3Mn+6Cr+Mo: 8.1% max, and
Mn+6Cr+10Mo: at least 3.5%
The Omiya et al patent is very clear that an initial heat-treating (soaking) step is conducted at a temperature of at least 780xc2x0 C. (1436xc2x0 F.). See column 5, lines 64-67; col 6, lines 2-4: xe2x80x9cIn order to obtain the desired microstructure and achieve stable formability, it is necessary to heat the steel sheet at 780xc2x0 C. or above, which is higher than the AC1 point by about 50xc2x0 C. . . . Heating should be continued for more than 10 seconds so as to obtain the desired microstructure of ferrite+austenite.xe2x80x9d The process description then goes on to say the steel sheet is cooled to the plating bath temperature (usually 440-470xc2x0 C., or 824-878xc2x0 F.) at an average cooling rate greater than 1xc2x0 C./second, and run through the plating bath. After plating, cooling at a rate of at least 5xc2x0 C./second will achieve the desired microstructure of predominantly ferrite and martensite. Optionally, the plated sheet may be heated prior to cooling, in an alloying procedure (often called galvannealing) after metal coating but prior to the final cooling.
Omiya et al clearly do not appreciate that it is possible to achieve a dual phase product without the high temperatures of their soaking step, or that a particular holding step following a lower temperature soak can facilitate the desired microstructure formation.
I have found, contrary to the above quoted recitation in the Omiya et al patent, that not only is it not necessary to maintain the initial heat treatment temperature at 780xc2x0 C. (1436xc2x0 F.) or higher, but that the desired dual phase microstructure can be achieved by maintaining the temperature during an initial heat treatment (soaking) in the range from AC1+45xc2x0 F., but at least 1340xc2x0 F. (727xc2x0 C.), to AC1+135xc2x0 F., but no more than 1425xc2x0 F. (775xc2x0 C.). One does not need to maintain the temperature at 780xc2x0 C. or higher, contrary to the Omiya et al patent, provided the rest of my procedure is followed. For convenience hereafter, my initial heat treatment will be referred to as the xe2x80x9csoak.xe2x80x9d However, my process does not rely only on a lower temperature for the soak as compared to Omiya et al; rather, the soak temperature of (AC1+45xc2x0 F.) to 1425xc2x0 F., usually 1340-1420xc2x0 F., must be coupled with a subsequent substantially isothermal heat treatment, termed the holding step, in the range of 850-920xc2x0 F. (454-493xc2x0 C.). In the holding step, the sheet is maintained at 850-920xc2x0 F. (454-493xc2x0 C.), sometimes herein expressed as 885xc2x0 F.xc2x135xc2x0 F., for a period of 20 to 100 seconds, before cooling to room (ambient) temperature. Cooling to ambient temperature should be conducted at a rate of at least 5xc2x0 C. per second. It is important to note, once again, that the Omiya et al patent says nothing about a holding step at any temperature or for any time in their thermal process. Furthermore, my work has shown that if a steel as defined in the Omiya et al patent is soaked within Omiya""s defined, higher, soaking range (for example 1475xc2x0 F.) and further processed through a thermal cycle including a holding step as described herein (850-920xc2x0 F.), the resultant steel will not achieve the desired predominantly ferrite-martensite microstructure but will contain a significant amount of bainite and/or pearlite.
I express the lower temperature limit of the soak step as xe2x80x9cAc1+45xc2x0 F., but at least 1340xc2x0 F. (727xc2x0 C.)xe2x80x9d, because virtually all steels of Composition A will have an AC1 of at least 1295xc2x0 F.
The steel sheet should have a composition similar to that of the Ochiya et al patent:
manganese, chromium and molybdenum should have the relationship:
Mn+6Cr+10Mo: at least 3.5%
For my purposes, the silicon content may be as much as 0.5%, and, preferably, carbon content is 0.03-0.12% although the Omiya et al carbon range may also be used. This composition, as modified, may be referred to hereafter as Composition A.
Thus my invention is a method of making a dual phase steel sheet comprising soaking a steel sheet at a temperature of in the range from AC1+45xc2x0 F., but at least 1340xc2x0 F. (727xc2x0 C.), to AC1+135xc2x0 F., but no more than 1425xc2x0 F. (775xc2x0 C.), for a period of 20 to 90 seconds, cooling the sheet at a rate no lower than 1xc2x0 C./second to a temperature of 454-493xc2x0 C., and holding the sheet at temperatures in the range of 850-920F (454-493xc2x0 C.) for a period of 20 to 100 seconds. The holding step may be prior to the hot dip or may begin with the hot dip, as the galvanizing pot will be at a temperature also in the range 454-493xc2x0 C. (850-920xc2x0 F.). Immediately after the holding step, whether or not the sheet is galvanized, the sheet can be cooled to ambient temperature at a rate of at least 5xc2x0 C./second. Alternatively, after the sheet is coated, the sheet may be galvannealed in the conventional mannerxe2x80x94that is, the sheet is heated for about 5-20 seconds to a temperature usually no higher than about 960xc2x0 F. and then cooled at a rate of at least 5xc2x0 C./second. My galvannealed and galvanized thermal cycles are shown for comparison in FIG. 6.
The actual hot dip step is conducted more or less conventionallyxe2x80x94that is, the steel is contacted with the molten galvanizing metal for about 5 seconds; while a shorter time may suffice in some cases, a considerably longer time may be used but may not be expected to result in an improved result. The steel strip is generally about 0.7 mm thick to about 2.5 mm thick, and the coating will typically be about 10 xcexcm. After the holding and coating step, the coated steel may be either cooled to ambient temperature as described elsewhere herein or conventionally galvannealed, as described above. When the above protocol is followed, a product having a microstructure comprising mainly ferrite and martensite will be obtained.
Commercially, it is common to perform hot dip galvainizing substantially contiunously by using coils of steel strip, typically from 1000 to 6000 feet long. My invention permits more convenient control over the process not only because the soak step takes place at a lower temperature, but also because the strip may be more readily kept at the same temperature as the hot dip vessel entering and leaving it, with little concern about significant heat transfer occurring between steel strip and zinc pot that could heat up the molten zinc and limit production.
As applied specifically to a continuous steel strip galvanizing line, which includes a strip feeding facility and a galvanizing bath, my invention comprises feeding a cold rolled coil of steel strip of Composition A to a heating zone in the galvanizing line, passing the strip through a heating zone continuously to heat the strip to within the range of AC1+45xc2x0 F., but at least 1340xc2x0 F. (727xc2x0 C.), to AC1+135xc2x0 F., but no more than 1425xc2x0 F. (775xc2x0 C.), passing the strip through a soaking zone to maintain the strip within the range of AC1+45xc2x0 F., but at least 1340xc2x0 F. (727xc2x0 C.), to AC1+135xc2x0 F., but no more than 1425xc2x0 F. (775xc2x0 C.), for a period of 20 to 90 seconds, passing the strip through a cooling zone to cool the strip at a rate greater than 1xc2x0 C./second, discontinuing cooling the strip when the temperature of the strip has been reduced to a temperature in the range 885xc2x0 F.xc2x135xc2x0 F., but also xc2x130 degrees F. of the temperature of the galvanizing bath, (preferably within 20 degrees F.xc2x1the temperature of the bath, and more preferably within 10 degrees F.xc2x1the temperature of the bath), holding the strip within 30 degrees F. of the temperature of the galvanizing bath (again preferably within 20 degrees F.xc2x1the temperature of the bath, and more preferably within 10 degrees F.xc2x1the temperature of the bath) for a period of 20 to 100 seconds, passing the strip through the galvanizing bath, optionally galvannealing the coated strip, and cooling the strip to ambient temperature. The galvanizing bath is typically at about 870xc2x0 F. (850-920xc2x0 F.), and may be located at the beginning of the holding zone, or near the end of the hold zone, or anywhere else in the holding zone, or immediately after it. Residence time in the bath is normally 3-6 seconds, but may vary somewhat, particularly on the high side, perhaps up to 10 seconds. As indicated above, after the steel is dipped into and removed from the zinc bath, the sheet can be heated in the conventional way prior to cooling to room temperature to form a galvanneal coating, if desired.